1. Field of the Invention
The present invention relates to phase shifters for electromagnetic waves and, more particularly, to RF phase shifters, delay lines and the like which may be controlled electrically.
2. Description of the Prior Art
In many fields of electronics, it is often necessary that the phase of an electronic signal be controlled such that an output signal has some desired phase relation with respect to the input signal. In the field of RF signal transmission, phase shifter waveguides have been routinely employed for adjusting the phase of a particular electromagnetic field component at an output relative to the phase of that field component at the input. When the RF signal to be phase shifted is in the microwave or millimeter wave band, it is customary to employ ferrite phase shifters in waveguide transmission circuits to do the phase shifting job. These devices usually accomplish phase shifting by varying the transmission delay or transit time of the RF signal over a predetermined distance and, therefore, varying the phase of the signal as it passes through the ferrite. Although such devices have served the purpose, they have not proven entirely satisfactory under all conditions of service for the reasons that considerable difficulty has been experienced with phase shift sensitivity in extreme temperature conditions and difficulties encountered due to hysteresis effects. These problems, inherent in ferrite materials, usually result in substantial power losses, signal distortions, noise, etc.
More specifically, a ferrite phase shifter is a two-port RF transmission line in which the phase of the output signal is varied by changing a d-c magnetic field in which the ferrite is immersed. Phase shifts up to 360 degrees are obtainable in a relatively small structure. However, unwanted variations in the output phase due to temperature changes or ambient magnetic fields or both have often required that these devices be contained in a controlled environment. In some situations, the inconvenience of a temperature-controlled environment may be eliminated with a feedback control loop about the ferrite phase shifter to precisely control the phase shift. In either case, the additional costs necessary to mitigate the adverse affects of temperature and other hostile ambient conditions, and the attendant loss in phase shift sensitivity have led designers of RF phase shifters to look elsewhere for better solutions to these critical problems. Consequently, those concerned with the development of electronically controlled phase shifters have long recognized the need for more reliable, more sensitive and less costly RF phase shifters. The present invention fulfills this need.